1. Field of the Invention
This invention concerns an improved process and apparatus for producing chlorine and iron oxide from iron chlorides. More particularly, it concerns such a process and apparatus wherein the ferric chloride is treated in the vapor phase with oxygen in a multistage recirculating-fluidized-bed reactor.
2. Prior Art
Several industrial processes, such as the direct chlorination of ilmenite ores to produce titanium dioxide, generate large quantities of by-product iron chlorides. Disposal of these iron chlorides poses potential pollution problems and represents an economic loss for the chlorine contained in the iron chlorides.
A multistage recirculating-fluidized-bed reactor, in which iron chlorides are converted to chlorine and iron oxide, is suggested by Reeves et al., U.S. Pat. No. 3,793,444. The reactor is divided into zones by foraminous members, which have a plurality of holes through which gas and particulate matter flow. Each zone operates with an upward superficial gas velocity in the range of 0.2 to 2 feet/second [0.06 to 0.6 meters/second] and contains a dense fluidized mass of bed particles containing iron oxide. Reeves et al. discloses a reactor in which the zones are of equal cross-sectional dimensions, as well as a reactor in which the cross-sectional dimensions are increased from zone to zone as one proceeds form the inlet to the outlet of the reactor. Entrained solids exiting the reactor are pneumatically conveyed to downstream equipment for separating the gases from the solids and recycling a portion of the solids to the fluidized bed of the reactor. It has now been found that when such a reactor is operated on an industrial scale, the foraminous dividers provide undesired additional pressure drop in the system and have a tendency to become plugged.
Harris et al., "Process for Chlorination of Titanium Bearing Metals and for Dechlorination of Iron Chloride" in World Mining and Metals Technology, Alfred Weiss, e.d., The Society of Mining Engineers, New York, Chapter 44, pages 693-712, August, 1976, reports that the Bureau of Mines has been investigating a process in which ferric chloride in the vapor phase is treated with oxygen in a single-stage, dense fluidized-bed reactor. The process is described in greater detail by Paige et al., "Recovery of Chlorine and Iron Oxide from Ferric Chloride," Journal of Metals, pages 12-16 (November, 1975). In this process, preheated oxygen is fed to the bottom of a heating section which contains a dense fluidized bed of iron oxide and wherein an optional fuel may be burned. Ferric chloride is screw-fed to the top of the heating section which is connected to the bottom of the reaction zone. The reaction zone is of larger cross section than the heating zone. The ferric chloride is vaporized and then reacts with the oxygen as both pass through the dense fluidized-bed reaction zone of the reactor. Off-gases are passed from the reaction zone to a cyclone separator to remove any entrained solids. In a separated stream, a portion of the iron oxide bed material and product, which is collected at the bottom of the heating section, is treated with sodium chloride catalyst and then recycled to the reactor in an amount equal to about 25% by weight of the ferric chloride feed. The authors point out that this process operates satisfactorily on a laboratory scale, but that an industrial process for conversion of waste ferric chloride by dechlorination is still needed.
Although fluidized-bed reactors which operate with a dilute phase (e.g., with solids fraction below 0.05 and with superficial gas velocities of about 15 to 30 feet/second [4.5 to 9 meters/second] are known, such reactors have not been utilized for the conversion of iron chlorides to chlorine and iron oxide. We have found that such single-stage dilute-phase reactors would be inadequate for converting iron chlorides to chlorine and iron oxide on an industrial scale because of difficulties in supplying sufficient heat for maintaining reaction temperature control and in providing high conversions at high throughputs.